Toner concentration and auto bias control apparatus

ABSTRACT

A toner concentration and auto bias control apparatus includes a circuit for producing a signal which is a function of the charged density of an electrostatic image passing a predetermined position of a path, a toner concentration control apparatus responsive to such signal for controlling the toner concentration of a mixture of toner and carrier and an auto bias circuit responsive to such signal for adjusting the potential of a developed electrode to influence the toning process.

United States Patent 11 1 1111 3,779,204 Altmann 1 Dec 18, 1973 TONER CONCENTRATION AND AUTO 3,348,522 10/1967 Donohue 118/637 x BIAS C T APPARATUS 3,367,307 2/1968 Lawes et a1 1 18/7 X 3,406,334 10/1968 Marquart et al.... 324/72 X [75] Inventor: Conrad Altm n, c er NY. 3,449,658 6/1969 Robinson et a1 324/72 x Assigneez Eastman Kodak p y, 3,674,353 7/1972 Trachtenherg 1 355/3 DD Rochester, NY Primary ExaminerMorris Kaplan [221 Fled: 1972 Att0rneyW. H. J. Kline et a1. [21] Appl. No.: 280,397

[57] ABSTRACT E3 8 118/8 1 18/ 32 A toner concentration and auto bias control apparatus 1 l!- g inc udes a circuit fo producing a Si n s a [58] Field of Search 118/7 8 637 DIG 24' g W17 5 ;l'f' 3 3 function of the charged density of an electrostatic l image passing a predetermined position of a path, a toner concentration control apparatus responsive to [56] Reterences cued such signal for controlling the toner concentration of a UNITED STATES PATENTS mixture of toner and carrier and an auto bias circuit 2,956,487 10/1960 Giamo, Jr. 118/637 UX responsive to such signal for adjusting the potential of ,0 ,4 63 Cod chini l 1 X a developed electrode to influence the toning process.

3,094,499 6/1963 Snelling 324/32 X 3,161,882 12/1964 Mullin 117/17.5 X 4 Claims, 3 Drawing Figures PATENTEU DEC] 8 I975 SHEET 1 [IF 2 PAIENIEDnEc 18 ms 3.779.204

sum 2 OF 2 FROM 8 4 I THRESHOLD TO 05c TECTOR 52 DELAY CIRCUIT FIG. 3

TONER CONCENTRATION AND AUTO BIAS CONTROL APPARATUS BACKGROUND OF THE INVENTION 1. Field Of The Invention This invention relates to electrographic development apparatus, and more particularly, to toner concentration and auto bias apparatus for maintaining the relative concentration of toner in a mixture of toner and carrier components at a level wherein copies of uniformly good density are consistently producable.

2. Description Of The Prior Art In electrographic apparatus, an electrostatic image, formed on the surface of a drum or web, is developed by application of finely divided toner particles thereon to form a toner image. In certain electrogrphic apparatus, toner images are formed from electrostatic images by brushing a developer mixture of'ferromagnetic carrier particles and much smaller toner particles (typically suitably pigmented or dyed resin-based particles) across the electrostatic images. The contact of the ferromagnetic particles with the toner particles charges the toner particles by triboelectrification to a polarity needed so that the toner particles are attracted to the electrostatic images for toning. In this process, toner particles are depleted from the developer mixture, requiring subsequent replenishment to avoid a gradual reduction in density of the toner images. Toner replenishment is accomplished by several different types of apparatus. In one type, a given amount of toner is added to the mixture after a given number of copies is made. This approach is acceptable providing the amount of toner used for each copy is reasonably predictable. In apparatus which do not tone broad, solid areas, however, such as many magnetic brush developing cascade toning apparatus the amount of toner used in any copy of group of copies can vary substantially. For this reason, toner concentration monitors have been designed which automatically add toner according to the results of a monitoring process; see for example U.S. Pat. No's 2,956,487 to Giamo; 3,094,049 to Snelling; 3,227,549 to Ullrich; 3,233,781 to Grubbs; 3,399,652 to Gauron; and 3,409,901 to- Dost et al.

Certain toner concentration monitors examine the quality of a toner image before adding toner. These apparatus suffer from the disadvantage that toner is not added until only after copy quality is degraded.

In certain optical monitoring apparatus a reduction in the relative toner concentration in the mixture below a desired level can be detected in the mixture of toner and carrier before image degradation. (See for example, U.S. Pat. No.s 3,233,781 to Grubbs and 3,409,901 to Dost etal). Such optical apparatus have not proven entirely satisfactory in operation, especially over extended periods of time. This may be attributed, at least in part, to the lighting source utilized for illuminating the developer mixture which may be a luminous energy source of relatively low output, unstable intensity and short life, and further to the fact that spectral characteristics of light from the source may be closely akin to the background light, thereby making background discrimination difficult. Also contributing to the instability of such devices is the fact that the photoconductive cell used for detecting developer reflectance may be sensitive to temperature and humidity variations of the type encountered in normal machine operation, or may exhibit an undesirable hysteresis or light history effect.

itor wherein monitoring is achieved by sensing the charge of an image before and after development. The monitor produces an analog signal representative of the difference beween, before and after image charges. Although such an approach can perform satisfactorily, it is difficult in practice to correlate the analog signal in terms of toner lost to the development station.

In another approach to improve toning often referred to as Auto-Bias, the potential of an electrode in the development station is adjusted as a function of the charge density of the electrostatic image which is sampled upstream from the development station. Various aspects of such approach are described in detail in U.S. Pat. No. 2,956,487 and 3,611,982.

SUMMARY OF THE INVENTION An object of the present invention then is to improve elcctrographic development by providing an apparatus for automatically maintaining the toner concentration and provides an auto bias function to produce high quality toner images.

Another object of the invention is to provide an inexpensive, yet highly accurate and reliable apparatus for automatically controlling the level of toner concentration in an electrographic developer and adjusting auto bias as a function of a single signal.

In the disclosed apparatus, there is provided means for producing a signal which is a function of the charge density of electrostatic images passing a predetermined position along a path upstream from a development station. The apparatus further includes means responsive to said signal for causing toner to be added to the mixture to increase the relative concentration of the toner component and obtain a desired relative concentrations of mixture components and means responsive to said signal for adjusting the potential bias of an electrode in said development station.

It is a feature of the invention that the same signal is utilized to control auto bias and toner concentration and thus provide a noise compensation" function if there should be unwanted noise components in such signal.

Other objects of the invention and its various advantages will become apparent from the ensuing detailed description of the preferred embodiment shown below.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic vertical section of an apparatus embodying the invention; and FIGS. 2 and 3 are circuit diagrams partially in schematic and partially in block form which may be utilized with the apparatus shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT For a general understanding of an electrographic reproducing or copier apparatus wherein the invention has particular utility, reference is made to FIG. 1 wherein various stations of an electrophotographic apparatus are schematically illustrated. As in most electrophotographic copiers, an information medium 13 such as a document is illuminated by radiation from flash lamps 14. Such radiation is reflected from the medium and projected by a lens 15 onto a photosensitive member 116, shown as a web, to selectively dissipate change and form an electrostatic image. The web had previously been uniformly charged at a charging station 20. The photosensitive member 16 includes a photoconductive layer with a conductive backing. The photoconductive layer may comprise, for instance, a heterogeneous mixture of a bisphenol A-polycarbonate binder, a triarylmethane organic photoconductor and a pyrylium sensitizing dye. The apparatus 10 further ineludes a development station 22 at which the electrostatic image is contacted with finely divided charged toner particles that adhere to the photoconductive layer in a configuration defined by the electrostatic image; a transfer station 2d in which toner particles are transferred in the image configuration to a receiving surface of a copy sheet 216 on which it can be subsequently permanently fused; and a cleaning station (not shown) in which residual toner is removed from the photoconductive layer so that it can be reused. In certain known modifications of this same system, one or more of these stations may be eliminated. For a more complete description of an electrographic apparatus reference may be made to commonly assigned copending patent application Ser. No. 191,304 filed Sept. 21, 1971 now U.S. Pat. No. 3,698,805, entitled CON- TROL APPARATUS FOR ELECTROPHOTO- GRAPHIC APPARATUS to Hickey et al. An example of an exemplary development station is set forth in U.S. Pat. No. 3,543,720 to Drexler. The development station 22 shown is as a magnetic brush-type apparatus which includes a cylindrical brush member 30 which is rotated in a counterclockwise direction as shown by the arrow, in a magnetic field produced by magnets (not shown). The brush member 30 is effective to transport a portion of a mixture 33 of ferromagnetic carrier particles and toner particles from a developer reservoir 34 which is a conductive member and acts as an electrode across the surface of the web 16 such that toner particles will contact a portion of and then remain on charged image areas on the surface of the web 16. Since a mixture having a high concentration toner would tend to clog, a continuous control mixing paddle-wheel 35 has been provided to uniformly blend the mixture.

As the relative concentration of toner in the mixture decreases, the resulting developed toner images will tend to have a lower density and eventually will appear washed-out. In order to alleviate this condition, a toner concentration monitor has been provided which is shown generally at 410 and is coupled to a charge sensor (disposed at a predetermined position along the web path) such as an electrometer 4-7 which is coupled to a source of potential shown for convenience of illustration as a battery 48. The probe d7 is also coupled to a development station potential control apparatus 71 (also referred to herein as an auto bias system which is adapted to control the potential of the development reservoir 34 to provide the well known autobias function described in detail in U.S. Pat. No. 2,956,487. An

example of an electrometer suitable for use with the invention would be an induction plate or probe fabricated from a highly conductive material such as copper or aluminum, and spaced closely to the web to web to receive an induced charge as an electrostatic image passes by. The electrometer 47 may extend over the whole width of the area and is adapted to scan the electrostatic image and produce an electrostatic signal which is proportional to the charge density of that portion of the electrostatic image passing under the probe 47. In placing the probe 47, it is necessary to take into account the factor that some photoconductive mem bers exhibit a sharp reduction in conductivity for a short time period, usually less than one second, following exposure. Thus, it is desirable to displace the electrometer 47 a sufficient distance from the exposure station 21 so that the effect of such conductivity reduction will have run its course before the undeveloped image arrives at the meter 47. The probe 47 is directly coupled to the auto-bias system 711 which will be described in connection with FIG. 3.

The signal produced by the probe 47 is also applied to the circuitry 50. The circuitry 50 is effective when the toner concentration of the mixture will be reduced below a desired level to actuate a solenoid device 52 which rotates a metering wheel 57 releasing a predetermined increment of toner from a toner container 56.

Such toner increment, under the influence of gravity, falls into the reservoir 34 wherein the paddle-wheel 35 uniformly blends it into the developer mixture 33, thereby returning the relative concentrations of toner and carrier components to a desired level. One of the advantages obtainable with the toner control apparatus 40 is that any conventional toner replenisher mechanism can be actuated by the aforementioned output signal, such as for example that disclosed in U.S. Pat. No. 3,409,901.

Turning now to FIG. 2, wherein the circuit 50 for actuating the solenoid 52 is shown in more detail and ineludes an integrator 60 shown as an operational amplitier with a feedback capacitor 62 coupled between its output and non-inverting input terminals,

The inverting input of the operational amplifier 60 is coupled to ground. The output signal produced by the amplifier 60 is applied to a charging circuit comprising an adjustable resistor 66 and a charging capacitor 66 coupled to ground. At the electrical junction of the re sistor 64 and the capacitor 66 there is provided a threshold detector circuit 6% which may include a conventional unijunction transistor which fires when the capacitor 66 reaches a predetermined charge level. At such time, the threshold circuit 60 will provide a signal to the solenoid 52 causing its actuation.

Briefly, in operation, the circuit 50 functions as follows. The charge level of an electrostatic charge density (viz., field strength) is continuously sampled and the output of the operational amplifier 60 provides a signal representative of the cumulative total charge level of electrostatic images passing by the probe 67. When a predetermined charge level on the capacitor 66 is reached (which may be changed by adjusting the resistance of the resistor 64) the threshold detector circuit 66 provides a pulse type signal which actuates the solenoid 52 causing an increment of toner to fall into the mixture 33 to return the relative concentration of the toner in the mixture to a desired level. The pulse signal also actuates a relay 70 which closes relay contacts 70a for a sufficient time period to dissipate the charge on the capacitor 66 and reduce the output potential of the amplifier 60 to ground level. The relay 70 is shortly thereafter de-energized and opens contacts 70a thereby conditioning the circuit 50 to repeat the above operation.

A secnd embodiment of the invention is shown in P16. 1 wherein a probe 47' (shown in dotted lines) is disposed in a predetermined position along the web path disposed after the development station 22. Assuming the electrostatic images have a negative potential, then toner images corresponding thereto will have positive potential. Accordingly the probe 47 provides a similar function as the probe 47 but will provide a signal to the circuitry 50 which is representative of the charge density of deposited toner. The integrator 60 now provides a signal representative of the total change of toner and the threshold detector 68 will be actuated when a predetermined amount of toner has been lost to the reservoir 34.

The auto-bias system 71 is shown in FIG. 3 and includes a preamplifier 72 which receives a signal directly from the probe 47 and amplifiers same. The preamplifier 72 also provides an isolation function. A conventional delay circuit 74 receives an input from the amplifier 72 and is adapted to delay the output signal of the amplifier 72 for a time-period sufficient to permit an electrostatic image sampled by the probe 47 to advance over the development station for toning. An example of such a circuit is set forth in U. S. Pat. No. 3,61 1,982. The output of the delay circuit 74 is applied through an adjustable resistor 75 to a power amplifier 76 which adjusts the output signal to a potential level suitable for use and applies it to the reservoir or electrode 34.

It will be appreciated by those skilled in the art that the subject invention has utility in a number of electrographic reproduction systems, including xeroprinting, thermoxerography, and'xerothermography, and in no way is dependent upon the physical form of the developer (viz. liquid or particulate) or the manner in which it is applied to the electrostatic image to render it visible.

The invention has been described in detail with particular reference to a preferred embodiment thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

1 claim:

1. For use in electrographic apparatus having a member charged with a pattern forming an electrostatic image, developing means for contacting such electrostatic images with a mixture of toner and carrier components to form toner images corresponding to the electrostatic images the development means including an electrode the potential applied to which is adjustable for influencing toning and actuable toner replenishment means effective when actuated for adding toner to the mixture to compensate for toner depletion resulting from form ing the toner images, an improved toner replenishment control apparatus comprising:

a. means for sensing the charge density on electrostatic images prior to toning and for producing a signal the magnitude of which is representative of the charge density thereof passing a predetermined position;

b. means responsive to said signal for actuating the toner replenishment means to cause the addition of toner to the mixture; and

0. means responsive to said signal to adjust the potential of the electrode.

2. ln electrographic apparatus in which a surface carrying an electrostatic charge pattern is contacted at a development station by a developer mixture of carrier particles and electrostatically attractable toner particles to form a toner image on the surface according to the pattern, a toner concentration and auto bias control apparatus comprising:

a. first means for sensing the electric field strength associated with an electrostatic charge pattern to provide a signal representatative thereof; and

b. second means responsive to said signal for indicating'the amount of toner which will be depleted by producing a second signal the magnitude of which is a function of the amount of toner which will be deposited on the surface;

0. an adjustable potential electrode disposed at the development station; and

d. third means responsive to said signal for adjusting the potential applied to said electrode.

3. The invention as set forth in claim 2 including:

e. a container for storing toner particles;

f. actuable means effective when actuated for causing toner from said container to be delivered to the mixture; and

g. said second means including integrator means responsive to said second signal for actuating said actuable means.

4. The invention as set forth in claim 3 wherein said third means includes a delay circuit. 

1. For use in electrographic apparatus having a member charged with a pattern forming an electrostatic image, developing means for contacting such electrostatic images with a mixture of toner and carrier components to form toner images corresponding to the electrostatic images the development means including an electrode the potential applied to which is adjustable for influencing toning and actuable toner replenishment means effective when actuated for adding toner to the mixture to compensate for toner depletion resulting from forming the toner images, an improved toner replenishment control apparatus comprising: a. means for sensing the charge density on electrostatic images prior to toning and for producing a signal the magnitude of which is representative of the charge density thereof passing a predetermined position; b. means responsive to said signal for actuating the toner replenishment means to cause the addition of toner to the mixture; and c. means responsive to said signal to adjust the potential of the electrode.
 2. In electrographic apparatus in which a surface carrying an electrostatic charge pattern is contacted at a development station by a developer mixture of carrier particles and electrostatically attractable toner particles to form a toner image on the surface according to the pattern, a toner concentration and auto bias control apparatus comprising: a. first means for sensing the electric field strength associated with an electrostatic charge pattern to provide a signal representatative thereof; and b. second means responsive to said signal for indicating the amount of toner which will be depleted by producing a second signal the magnitude of which is a function of the amount of toner which will be deposited on the surface; c. an adjustable potential electrode disposed at the development station; and d. third means responsive to said signal for adjusting the potential applied to said electrode.
 3. The invention as set forth in claim 2 including: e. a container for storing toner particles; f. actuable means effective when actuated for causing toner from said container to be delivered to the mixture; and g. said second means including integrator means responsive to said second signal for actuating said actuable means.
 4. The invention as set forth in claim 3 wherein said third means includes a delay circuit. 